NES

Nestopia is the current king of the NES emulation hill. It uses highly optimized cycle exact emulation, allowing it to run titles that rely on precise timing, many of which break under inferior emulators. This accurate emulation does come at a cost; Nestopia requires a minimum of a 600MHz machine for full speed; but if you have one, this the best available NES emulation for Mac.

OpenEmu is an open source project to bring game emulation to OS X as a first class citizen, leveraging modern OS X technologies such as Cocoa, Core Animation and Quartz, and 3rd party libraries like Sparkle for auto-updating. OpenEmu is based on a modular architecture, allowing for game emulators as plugins, this means OpenEmu can support a host of different emulation engines and back-ends while retaining a familiar OS X native front-end.